1. Field of the Invention
The present invention relates to a radio communication system that employs an adaptive modulation scheme, and more particularly to a radio communication system and a radio communication terminal for the same equipped with a scheduler by which a base station selects a radio communication terminal as a communication destination in response to a communication request from a radio communication terminal.
2. Description of the Related Art
A radio communication system that employs a so-called adaptive modulation and coding adaptively controlling a modulation scheme and a channel coding rate performs transmission control between a base station and a radio communication terminal as follows. That is, the radio communication terminal firstly measures reception qualities of downlinks of a plurality of neighboring base stations, respectively, to select a base station with the highest reception quality on the basis of the measured result. The communication system decides a usable transmission format, i.e., a combination of the modulation scheme and the channel coding rate, under the reception quality of the downlink of the selected base station and transmits the decided transmission format as a channel quality indication (CQI) to the selected base station through an uplink. In response to this, the base station uses a dedicated channel for the communication terminal to change over the modulation scheme and the channel coding rate of information data. In this manner, when transmitting the information data from the base station to the communication terminal through the dedicated channel, the base station is able to transmit the information data by using the high data rate that error resistance is low in the high SNR condition at the communication terminal, and in contrast, in the low SNR condition, it becomes possible to transmit the information data by using the low data rate that error resistance is high at the communication terminal.
To perform the forgoing transmission control, the communication terminal or the base station is provided with a table. The table stores a predicted down data communication rate by associating with the reception quality of the downlink. The predicted downlink data rate directly indicates an extremely accurate data rate corrected on the basis of, for instance, statistical data of an error rate, etc., of a predicted or past down data transmission. If the communication terminal is provided with the table, the communication terminal reads out the predicted down data rate corresponding to the down reception quality from the table to notify the predicted down communication rate to the base station. If the base station has the foregoing table, the base station reads out the predicted down data communication rate corresponding to the down reception quality toward the targeted radio communication terminal form the table, based on the CQI information transmitted from the radio communication terminal. As a result, a data communication between the base station and the communication terminal is performed at the communication rate corresponding to the reception qualities at times of the down channel.
By the way, a service form of the communication system employing the aforementioned adaptive modulation and coding rate is generally a best-effort type. Therefore, the communication terminal requires communication only with the base station having the best reception quality of the downlink. The base station transmits a packet of the information data on a preferential basis to a radio communication terminal excellent in the reception quality of the downlink and requiring a high-data rate. A terminal selecting (scheduling) algorithm used by the base station in this case is called a maximum CIR. In the maximum CIR, a radio communication terminal not excellent in the low SNR condition becomes to be placed a low priority to communicate with the base station.
A 1×EV-DO system, which conforms to, for instance, technical specifications “C. S0024 cdma2000 High Rate Packet Data Air Interface Specifications” presented by the standard-setting organization “3GPP2”, employs a proportional fairness (PF) scheduler as a scheduler to eliminate the forgoing failure and enhance both throughput seen from the base station side and throughput seen from the radio communication terminal side while keeping balance there between. The PF scheduler takes a data quantity which has been transmitted from the base station to the communication terminal in the past into account in addition to the reception quality of the downlink of the radio communication terminal (for instance, refer to IEEE international conference, VTC 2000 spring, announcement original copy, written by A. Japali, R. Padovani, and R. Pankaj, “Data throughput of CDMA-HDR a High Efficiently-High Data Rate Personal Communication Wireless System”).
For example, the base station calculates each evaluation function value “SNR_inst/SNR_ave” for each radio communication terminal, respectively. The “SNR_inst” is an instantaneous signal-to-noise ratio (SNR) of the down link notified from the radio communication terminal to the base station. The “SNR_ave” is an averaged value of the SNRs of the downlinks notified from the communication terminal to the base station in the past. With such a scheduler used, since the communication terminal, of which the downlink reception quality has become excellent, has an increased high potential for being selected, the communication system becomes possible to reduce inequality of throughput for a reception environment among radio communication terminals.
However, since the conventional 1×EV-DO system gives weight the throughput between the radio communication terminal and the base station, the communication terminal makes communications with the base station with the best down reception quality. Therefore, concentrating the communication requests from a large number of radio communication terminals to a specific base station is a possible risk. In such a case, regardless of the level of the possibility to be selected by the base station, all the radio communication terminals transmit the CQIs through uplinks, respectively, so that increases in signaling overhead in the uplinks and in terminal selection processing loads on the base station will be brought.
This problem becomes conspicuous in an orthogonal frequency division multiplex system capable of arranging a plurality of resource blocks even in frequency directions by every section hour.
That is to say, the OFDM system makes communications by using a large number of subcarriers simultaneously. At this moment, each subcarriers can be assigned to individual radio communication terminal. The subcarriers divided into groups are called, for instance, chunk and subcarriers are assigned in chunk to the respective communication terminals. If the aforementioned terminal selecting algorithm is employed to such an OFDM system, the communication terminals measure the reception quality of the downlink for each base station, respectively, to select the base station with the best reception quality. Each communication terminal transmits the measured result for each chunk as the CQIs to the selected base station. In response to this transmission, the bases station decides radio communication terminals to be assigned to each chunk on the basis of the notified CQIs to notify the chunks to be used and transmission formats for information data to the assigned communication terminals through the control channels. After this, in the dedicated channels in the downlinks, each communication terminal receives the information data in the notified transmission format through the notified chunk.
In this way, when the terminal selecting algorithm is employed, the CQIs are transmitted from the communication terminals to the base stations for each chunk. Therefore, the number of transmissions of the CQIs increases in response to the number of chunks, namely the number of resource blocks in the frequency direction. Thus, the signaling overhead on the uplinks from the communication terminals to the base stations increases then a reduction in throughput becomes further conspicuous.